Heat exchangers having cylindrical shells and helical tubes for heating fluid in the tubes are well known in the prior art. Generally, fluid flowing in the tubes is heated by flowing combustion gases provided by a burner located interior to the coils. U.S. Pat. No. 6,026,801 is directed to a coil heat exchanger having an interior plug. U.S. Patent Publication No. 2011/0041781 is directed to a coil tube heat exchanger having a dual-diameter outer cylindrical housing, a buffer tank within the helix coil of the heat exchanger and a rope seal disposed between adjacent coil loops of a portion of the helix coil. U.S. Pat. No. 7,523,721 is directed to a heat exchanger that includes a partition that partitions the space surrounding a coil tube into first and second regions to increase heat exchange efficiency. U.S. Pat. No. 8,343,433 is directed to a tube reactor having parallel reactor tubes.